Re: How to assign a non-const object to const reference

=?Utf-8?B?QWxhbWVsdQ==?= <>
Wed, 3 May 2006 08:21:04 -0700
Thank u so much Igor.
Was very much useful and informative.

When i followed the approach mentioned below, i didnt run into any error
const AClass& DerivedClass::Get()
return m_ObjA;

const AClass objA;
objA = derived.Get(); // Didnt get any error at this line.

Why is that so? Compiles, builds and works fine.

"Igor Tandetnik" wrote:

Alamelu <> wrote:

But Bruno...
if i modify the same function as below.. it doesn't give any error..
const AClass& DerivedClass::Get()
   return m_ObjA;
But my question is, even again here, it's equivalent to assigning a
non-const object to const reference. But why doesnt the complier
throw error here?

You are confusing two completely distinct situations: binding a
reference to an object, and assigning to an already-bound reference
(which is equivalent to assigning to an underlying object).

A reference must be bound to an object when it is created. Once bound,
it cannot be re-bound to a different object (aka reseated) - it becomes
an alias to an original object.

With this in mind, consider:

void DerivedClass::Get (const AClass &refA)
    refA = m_objA;
const AClass objA;

At the point of invocation of Get(), a reference refA is bound to the
function argument objA, and becomes just another name for this object. A
statement in the body of the function is effectively equivalent to objA
= m_objA, which is illegal since objA is const.

const AClass& DerivedClass::Get()
    return m_ObjA;

Here, the reference is a return value. A reference is created only when
the function returns, and is bound to the return value of the function.
No assignment takes place here: the function produces an (unnamed)
reference bound to m_ObjA. An assignment, if any, must take place
outside the function, at the call site. And of course, if the caller
does something like this

const AClass objA;
objA = derived.Get();

they'll get the same error as in the first case.
With best wishes,
    Igor Tandetnik

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